This study will address two major questions: 1. to what extent do modifiable factors e.g. nutrition and physical activity and nonmodifiable factors e.g. anthropometrics and reproductive hormone levels, determine peak bone mass; and 2. regardless of origin, do these determinants act through a common estrogen-dependent pathway. Specifically we will determine whether the increase in bone gain due to calcium and supplementation between ages 12-14, 14-16, or 12-16, results in increased peak bone mass. Determine how adolescent bone gain is related to: a. cumulative dietary intake; b. physical activity and physical fitness patterns; c. urinary collagen cross link excretion; d. changes in body composition and anthropometrics and e. late adolescent and early adult reproductive hormone levels. We will quantify the relationships between peak bone mass and a. menstrual history from age 12-21; b. cumulative estrogen exposure and c. cumulative calcium intake.